1. Field of the Invention
The invention generally relates to memory technology. In particular, the invention relates to non-volatile magnetic memory.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM), such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices. DRAM devices and SRAM devices are volatile memories. A volatile memory loses its data when power is removed. For example, when a conventional personal computer is powered off, the volatile memory is reloaded through a boot up process. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data for long periods of time when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), flash memory, and the like. Disadvantageously, conventional nonvolatile memories are relatively large, slow, and expensive. Further, conventional nonvolatile memories are relatively limited in write cycle capability and typically can only be programmed to store data about 10,000 times in a particular memory location. This prevents a conventional non-volatile memory device, such as a flash memory device, from being used as general purpose memory.
An alternative memory device is known as magnetoresistive random access memory (MRAM). An MRAM device uses magnetic orientations to retain data in its memory cells. Advantageously, MRAM devices are relatively fast, are nonvolatile, consume relatively little power, and do not suffer from a write cycle limitation. There are at least three different types of MRAM devices, including giant magneto-resistance (GMR) MRAM devices, magnetic tunnel junction (MTJ) or tunneling magneto-resistance (TMR) MRAM devices, and pseudo spin valve (PSV) MRAM devices. GMR MRAM devices separate at least two ferromagnetic layers with a metallic layer. In a MTJ MRAM device, at least two ferromagnetic layers are separated by a thin insulating tunnel barrier. A PSV MRAM device uses an asymmetric sandwich of the ferromagnetic layers and metallic layer as a memory cell, and the ferromagnetic layers do not switch at the same time.
In a conventional fabrication process, layers of materials that form a magnetoresistive sandwich for an MRAM cell body are formed by depositing a relatively large sheet of magnetoresistive materials. Conventional processes then selectively remove portions of the deposited sheet to form the MRAM cell bodies. Preferably, chemical etching techniques are used to selectively remove portions of deposited sheets. Examples of chemical etching techniques include dry etching techniques and wet etching techniques. However, such chemical etching techniques are not applicable to the fabrication of MRAM cells because the materials that are used to fabricate MRAM cells are relatively difficult to etch away with chemicals. For example, copper (Cu) is relatively difficult to remove by a chemical etching process.
Those in the art have resorted to ion beam milling or ion beam etching (IBE) processes to remove magnetoresistive materials from undesired areas. Ion beam milling is a physical milling process. A resist material is applied to regions that will form cells to protect or mask the regions from the effects of ion beam milling. Areas that are not protected by the resist are removed from the substrate assembly by bombardment with ions. The bombardment of ions sputters or peels away the unprotected material from the substrate assembly. Disadvantageously, ion beam milling operates with relatively low selectivity, and the portions of the substrate assembly that are near to the edges of the photoresist or the boundaries of an MRAM cell body can be easily damaged. The damage can result in a cell in which an edge of the cell and a center of the cell do not switch in unison. In addition, ion beam milling etch rates are relatively low, which results in relatively high costs and relatively low throughput rates.
A technique is needed to form MRAM cells without the disadvantages of ion beam milling.